The present invention relates to sensor characterization in an X-ray imaging system. In particular, the invention relates to storing sensor characterization in a memory resident in an X-ray image sensor.
X-ray image sensors comprising an array of active pixels are replacing film as the preferred tool for obtaining X-ray images of teeth and other body parts. An active pixel is a semiconductor device capable of converting an optical image into an electronic signal. When incident radiation interacts with a photosite, charge carriers are liberated and can be collected for sensing. The number of carriers collected in a photosite represents the amount of incident light impinging on the site in a given time period.
There are two basic devices, i.e., photodiodes and photogates, with many variants, employed to collect and sense charge carriers in a photosite. Variants of photodiodes include Pinned, P-I-N, Metal-Semiconductor, Heterojunction, and Avalanche. Photogate structures include Charge Coupled Devices (CCD), Charge Injection Devices (CID), and variants that include virtual phase, buried channel and other variations that utilize selective dopants which are used to control charge collection and transfer underneath and between the photogate(s) and the sense node.
Complementary metal-oxide semiconductor (CMOS) image sensors now also are available. Most CMOS imagers use Active Pixel Sensor (APS) technology, which utilizes an amplifier for each pixel. Due to process variations during the manufacture of these amplifiers, the actual gain and offset of each amplifier is slightly different from those of the other amplifiers. As a result, APS imagers suffer from high fixed pattern noise (FPN) problems. A solution to the variation in gain from one amplifier to another is to implement a unity gain amplifier (UGA) for every pixel. Each UGA requires the use of at least six field effect transistors (FETs).
An active column sensor (ACS) is described in U.S. Pat. No. 6,084,229. The ACS embodies the recognition that all but one of the FETs (the input FET) of each UGA for each pixel of a column in a CMOS active pixel sensor are redundant. In the ACS, the redundant FETs are absent, and replaced with a single shared UGA amplifier at each column for all the pixels in the column. The ACS has one dual input FET per pixel, and four or so shared FETs at each column.
Electronic image sensors, such as CCD or CMOS pixel sensors, have been adapted to be X-ray sensitive elements in dental and medical applications. The digital X-ray sensor is used to detect and record X-ray images which typically are downloaded to a personal computer via a cable. Examples of use of CCD-type and other X-ray image sensors in dental and/or medical environments are described in U.S. Pat. Nos. 5,671,738 and 5,744,806, which are incorporated herein by reference. An X-ray detector that comprises a plurality of CMOS active pixel sensors is described in U.S. Pat. Nos. 5,912,942 and 6,069,935.
Virtually all imaging sensors, CMOS, CCD, CID, etc., have defects, noise, etc., inherent in their design. One of the most common defects is dead pixels that do not record information. Other common defects include offset and variable sensitivity pixels, rows and columns. Typically, manufacturers of imaging sensors define an acceptable level of defects for their application, characterize these defects in production testing and create a software map (characterization) of the noise and defects of the sensor. This software map is then used in software filters to minimize the effect of the defects, so they do not appear in the images generated by the sensor. Without the filters, the dead pixels appear as black dots in the image.
In a typical X-ray imaging system in which image signals are supplied by an imaging sensor to a computer and processed by the computer before being displayed, a sensor characterization file corresponding to the sensor is stored on, for example, a floppy disk, and downloaded to such computer from the floppy disk. When the imaging sensor captures an image and supplies a corresponding image signal to the computer, the computer then applies a software filter using the sensor characterization file to eliminate the appearance of defects in the pixels and create an acceptable looking image which is displayed on a display or another medium. However, since users often forget the download step, images of poor quality frequently are obtained under those circumstances.